In a super-heterodyne receiver, signal frequencies are converted to a constant lower frequency (i.e., intermediate frequency) before detection. In the super-heterodyne receiver, a local oscillator (LO) signal in the receiver is mixed with incoming signals.
FIG. 1 shows the block diagram of a conventional super-heterodyne receiver 10. The receiver 10 includes a radio frequency (RF) amplifier 12, a first filter 14, a mixer 16, a first intermediate frequency (IF) amplifier 18, a surface acoustic wave (SAW) filter 20, a second IF amplifier 22, a variable gain function 24, a final IF amplifier 26, a second filter 28 and an analog to digital converter (ADC) 30.
In the receiver 10, the variable gain function 24 is placed after the SAW filter 20. As the power level at the input of the ADC 30 is increased, the gain is reduced, enabling a reduction in the dynamic range requirements of a final IF amplifier 26 and the ADC 30. Placing the variable gain function 24 after the SAW filter 20 has the advantage that the gain can be adjusted with respect to the input power of the ADC 30. The disadvantage, however, is that it does not reduce the dynamic range requirements of the mixer 16 or the first IF amplifier 18. Further, there is a limit to the dynamic range performance achievable with standard commercial mixer components.
If the variable gain function 24 is placed before the mixer 16, this would reduce the dynamic range requirements of the mixer 16 and the first IF amplifier 18. The difficulty with this approach is determining when to adjust the gain. Since an interfering signal may fall outside the pass-band of the SAW filter 28, the power of the ADC 30 can not be used to determine when the mixer 16 and the IF amplifiers would generate an unacceptable level of distortion.